1. Field of the Invention
The invention relates to a device for exposing the peripheral area of a wafer which is used to remove unnecessary resist on the wafer in the development process. The invention relates especially to a device for exposing the peripheral area of a wafer in which the peripheral area of a wafer can be exposed in a step shape and ring shape.
2. Description of Related Art
In the production of a semiconductor device, such as an IC, LSI or the like, photoresist, hereinafter called resist, is applied to the surface of a semiconductor wafer (hereinafter called a "wafer"), such as a silicon wafer or the like. In this way, a circuit pattern is formed.
The peripheral area of a wafer, however, is rarely used as an area in which the pattern is formed. In the case in which the resist is a positive resist, therefore, the peripheral area is not exposed, and as a result, the resist remains even after development in the peripheral area. This resist which has remained in the peripheral area causes impurities in peripheral devices and on the wafer surface, and thus, a reduction of yield due to loosening or the like during transport and holding of the wafer.
To remove the unnecessary resist in the peripheral area of a wafer in the development process, a process of exposure of the peripheral area of the wafer is undertaken in which, regardless of the process of exposure in the area in which the pattern is formed, the unnecessary resist is removed in the peripheral area.
To expose the peripheral area of a wafer usually the processes described below were carried out:
(1) The peripheral area of a wafer is irradiated with exposure light as the outlet end from which the exposure light emerges or the wafer is moved parallel to the wafer surface in orthogonally intersecting directions. The peripheral area of a wafer, as is shown in FIG. 14(a), is exposed in a step-shaped (hereinafter, this exposure process is called "stepped exposure"). (2) The peripheral area of a wafer is irradiated with exposure light as the wafer to which the resist has been applied is turned. The overall periphery or part of the periphery of the wafer is exposed in a ring-shape, as is illustrated in FIGS. 14(b) & (c) (hereinafter, this exposure process is called "ring-shaped exposure").
The stepped exposure described above in (1) is often used for exposure of the peripheral area when a circuit pattern is exposed in steps using a reduction projection exposure process of the stepper type on the wafer. In this stepwise exposure, several circuit patterns are formed on the wafer, each of which corresponds to a chip. The peripheral area of the region to be exposed in which the circuit pattern is formed is therefore step-shaped. This shape changes differently according to the exposure pattern.
In the peripheral area of the wafer, therefore, a stepped, unexposed area is formed. Therefore, the region with the unnecessary resist becomes stepped. This unnecessary resist causes a drop in yield due to loosening or the like, as was described above. The peripheral area of a wafer is therefore exposed by the process described above in (1) in steps so that creation of an area on the wafer which is not exposed is prevented.
Recently, there has been more and more often a demand for exposure of the peripheral area of a wafer in which, with simultaneous use of the stepped exposure and ring-shaped exposure, part of the peripheral area of the wafer is exposed in a step shape and the other part thereof is exposed in a ring shape, as is illustrated in FIG. 14(d).
To meet this demand, conventionally, an exposure device as described, for example, in Japanese patent disclosure document HEI 4-291938 for stepped exposure of the peripheral area of a wafer and a exposure device as described, for example, in Japanese patent disclosure document HEI 2-1114 (U.S. Pat. No. 4,899,195) for ring-shaped exposure of the peripheral area of a wafer were used, i.e. two exposure processes. Here, using two processes in which, using one of the devices some of the peripheral area of a wafer is exposed, for example, in a step shape, then the wafer is transported to the other device and the remainder of the peripheral area of the wafer is exposed in a ring shape, exposure of the entire peripheral area was produced.
However, the above described prior art has the following defects:
(1) It is necessary to have two exposure devices, i.e., one exposure device for stepped exposure of the peripheral area of a wafer and one exposure device for ring-shaped exposure of the peripheral area of a wafer. Therefore, the floor space occupied by the exposure devices in a clean room becomes larger.
(2) It is necessary, after placing the wafer on one of the exposure devices, to determine the seated state of the wafer and to position it, and after exposure, to transport the wafer to the other exposure device. Furthermore, it is necessary, afterwards, again to determine the seated state of the wafer and to position and expose it. There are, therefore, more working processes.
(3) To carry out the above described exposure in the same station, it can be imagined that using one exposure device for stepped exposure of the peripheral area of a wafer, the peripheral area of the wafer is exposed in a ring shape. In the stepped exposure in which exposure is performed along the outside edge of the circuit pattern, however, there is a requirement for exposure accuracy with high image resolution. Therefore, a large, heavy projection lens with high resolution is used. As a result, the exposure light exit end can, in principle, not move. Ring-shaped exposure using the conventional "copying method" in which the peripheral edge of a wafer is determined, and at the same time, the exposure light exit end is moved, therefore, can not be done.
On the other hand, in stepped exposure of the peripheral area of a wafer, if the width of the steps is reduced and the number of steps is increased, the area to be exposed can be made roughly ring-shaped. In this case, however, it is necessary to input a large amount of coordinate data. Data input is tedious. Furthermore, as a result of the enormously large amount of data, a storage device with a large capacity is required.
In addition, it can be considered a disadvantage that a long treatment time is needed if a MPU which enables fast treatment is not used.
If the resolution capacity of the above described data is reduced, the above described defects tend to be eliminated. Instead, however, microscopically small steps form on the border of the area exposed in a ring-shape, and proceeding from this border, area loosening of the resist occurs.
To eliminate the above described defects, one of the present applicants, together with others, has already proposed a device for exposure of the peripheral area of a wafer (Japanese patent disclosure document HEI 8-161443 and commonly assigned U.S. patent application Ser. No. 08/794,829). Here, in a first station, there is an irradiation part for step-shaped exposure, in a second station there is an irradiation part for ring-shaped exposure, the peripheral area of the wafer seated on the rotary support in the first station is exposed in a step shape, the rotary support is moved to the second station and the peripheral area of the wafer is exposed in a ring shape.
In the above described device, however, there was the disadvantage that a large device is needed because there is an irradiation part for stepped exposure and there is an irradiation part for ring-shaped exposure. In the first station, step-shaped exposure is performed, then the rotary support is moved to the second station and ring-shaped exposure is performed. Conventional ring-shaped exposure of the peripheral area is performed only after the wafer has been centered. Therefore, a centering device necessarily had to be used.